<!DOCTYPE html>
<html>

<head>
    <meta charset='utf-8' />
    <title></title>
    <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
    <script src='https://api.tiles.mapbox.com/mapbox-gl-js/v0.29.0/mapbox-gl.js'></script>
    <link href='https://api.tiles.mapbox.com/mapbox-gl-js/v0.29.0/mapbox-gl.css' rel='stylesheet' />
    <style>
        body {
            margin: 0;
            padding: 0;
        }

        #map {
            position: absolute;
            top: 0;
            bottom: 0;
            width: 100%;
        }
    </style>
</head>

<body>

    <style>
        .distance-container {
            position: absolute;
            top: 10px;
            left: 10px;
            z-index: 1;
        }

        .distance-container>* {
            background-color: rgba(0, 0, 0, 0.5);
            color: #fff;
            font-size: 11px;
            line-height: 18px;
            display: block;
            margin: 0;
            padding: 5px 10px;
            border-radius: 3px;
        }
    </style>

    <div id='map'></div>
    <div id='distance' class='distance-container'></div>

    <script src='https://api.mapbox.com/mapbox.js/plugins/turf/v2.0.2/turf.min.js'></script>
    <script>
        mapboxgl.accessToken = '<your access token here>';
        var map = new mapboxgl.Map({
            container: 'map',
            style: 'mapbox://styles/mapbox/streets-v9',
            center: [2.3399, 48.8555],
            zoom: 12
        });

        var distanceContainer = document.getElementById('distance');

        // GeoJSON object to hold our measurement features
        var geojson = {
            "type": "FeatureCollection",
            "features": []
        };

        // Used to draw a line between points
        var linestring = {
            "type": "Feature",
            "geometry": {
                "type": "LineString",
                "coordinates": []
            }
        };

        map.on('load', function () {
            map.addSource('geojson', {
                "type": "geojson",
                "data": geojson
            });

            // Add styles to the map
            map.addLayer({
                id: 'measure-points',
                type: 'circle',
                source: 'geojson',
                paint: {
                    'circle-radius': 5,
                    /* circle半径，单位像素 */
                    'circle-color': '#000' /* circle颜色 */
                },
                filter: ['in', '$type', 'Point'] /* 依据数据中type过滤 */
            });
            map.addLayer({
                id: 'measure-lines',
                type: 'line',
                source: 'geojson',
                layout: {
                    'line-cap': 'round',
                    /* 线条末端样式 */
                    'line-join': 'round' /* 线条相交处样式 */
                },
                paint: {
                    'line-color': '#000',
                    /* 线条颜色 */
                    'line-width': 2.5 /* 线条宽度 */
                },
                filter: ['in', '$type', 'LineString']
            });

            map.on('click', function (e) {
                var features = map.queryRenderedFeatures(e.point, {
                    layers: ['measure-points']
                }); /* 返回满足查询条件参数的可视化GeoJSON特性对象数组 */

                // Remove the linestring from the group
                // So we can redraw it based on the points collection
                if (geojson.features.length > 1) geojson.features.pop();

                // Clear the Distance container to populate it with a new value
                distanceContainer.innerHTML = ''; /* 清空html文本内容 */

                // If a feature was clicked, remove it from the map
                if (features.length) {
                    var id = features[0].properties.id;
                    geojson.features = geojson.features.filter(function (point) {
                        /* array.filter(callback):根据callback过滤元素组成新的数组 */
                        return point.properties.id !== id;
                    });
                } else {
                    var point = {
                        "type": "Feature",
                        "geometry": {
                            "type": "Point",
                            "coordinates": [
                                e.lngLat.lng,
                                e.lngLat.lat
                            ]
                        },
                        "properties": {
                            "id": String(new Date().getTime())
                        }
                    };

                    geojson.features.push(point); /* 新增元素 */
                }

                if (geojson.features.length > 1) {
                    linestring.geometry.coordinates = geojson.features.map(function (point) {
                        /* array.map(callback) :根据callback对数组元素进行变换*/
                        return point.geometry.coordinates;
                    });

                    geojson.features.push(linestring); /* 新增数组元素 */

                    // Populate the distanceContainer with total distance
                    var value = document.createElement(
                        'pre'); /* pre 元素可定义预格式化的文本。被包围在 pre 元素中的文本通常会保留空格和换行符。而文本也会呈现为等宽字体。 */
                    value.textContent = 'Total distance: ' + turf.lineDistance(linestring)
                        .toLocaleString() +
                        'km'; /* .textContent设置文本内容,toLocaleString():把数组转换为本地字符串。turf.lineDistance(geojson):这里猜测应该是turf.js库的函数 */
                    distanceContainer.appendChild(value); /*appendChild() 向元素内的最后添加子元素 */
                }

                map.getSource('geojson').setData(
                    geojson); /* getSource(id):获取对应的source，setData(data)：设置source上的data数据 */
            });
        });

        map.on('mousemove', function (e) {
            var features = map.queryRenderedFeatures(e.point, {
                layers: ['measure-points']
            });
            // UI indicator for clicking/hovering a point on the map
            map.getCanvas().style.cursor = (features.length) ? 'pointer' : 'crosshair'; /* 设置光标格式 */
        });
    </script>

</body>

</html>